UnassignedReferenceException: The variable UI of MenuManager has not been assigned.

image for error :

image for mono develop :

code in monodevelop :

 using UnityEngine;
 using System.Collections;
 using UnityEngine.SceneManagement;
 
 public class MenuManager : MonoBehaviour {
     public static MenuManager instance;
     public GameObject UI;
     public GameObject Controller;
     public GameObject LevelComplete;
     public GameObject GameOver;
     public GameObject GamePause;
     public GameObject Loading;
 
     void Awake(){
         instance = this;
         UI.SetActive (true);
         Controller.SetActive (true);
         LevelComplete.SetActive (false);
         GameOver.SetActive (false);
         GamePause.SetActive (false);
         Loading.SetActive (false);
     }
 
     public void Pause(){
         if (Time.timeScale == 1) {
             GamePause.SetActive (true);
             Time.timeScale = 0;
         } else {
             GamePause.SetActive (false);
             Time.timeScale = 1;
         }
     }
 
     public void Restart(){
         GlobalValue.checkpoint = false;
         GlobalValue.isUsingJetpack = false;
 
         Time.timeScale = 1f;
         Loading.SetActive (true);
         SceneManager.LoadScene (SceneManager.GetActiveScene ().buildIndex);
     }
 
     public void MainMenu(){
         GlobalValue.checkpoint = false;
         GlobalValue.isUsingJetpack = false;
 
         Time.timeScale = 1f;
         Loading.SetActive (true);
         SceneManager.LoadScene ("Menu");
     }
 
     public void ShowGameOver(){
         StartCoroutine (ShowMenu (1, GameOver));
     }
 
     public void ShowLevelComplete(){
         StartCoroutine (ShowMenu (1, LevelComplete));
     }
 
     public void NextLevel(){
         GlobalValue.checkpoint = false;
         GlobalValue.isUsingJetpack = false;
 
         Loading.SetActive (true);
         GlobalValue.levelPlaying++;        //
         SceneManager.LoadScene (SceneManager.GetActiveScene ().buildIndex + 1);
     }
 
     IEnumerator ShowMenu(float time,GameObject Menu){
         yield return new WaitForSeconds (time);
         Menu.SetActive (true);
     }
 
 }
 

Please help me :)